The White House says U.S. intelligence agencies have concluded "with varying degrees of confidence" that the Syrian regime has used chemical weapons, specifically sarin gas, on a small scale in Syria.

The intelligence assessment came in a letter the White House sent to lawmakers on April 25.

Defense Secretary Chuck Hagel first revealed the information in comments to reporters during a visit to the United Arab Emirates the same day.

"The U.S. intelligence community assesses with some degree of varying confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," Hagel said.

The letter -- the text of which was later released by the White House -- says the intelligence assessment was "based in part on physiological samples."

But it says further assessments are needed to establish "credible and corroborated facts."

"Given the stakes involved, and what we have learned from our own recent experience, intelligence assessments alone are not sufficient," the letter says.

Hagel, in his comments, also cautioned that "this is serious business."

"We need all the facts," he said.

President Barack Obama has warned that the use of chemical weapons in Syria would cross a "red line" that would require international action.

In its letter, the White House says it is ready to "respond appropriately to any confirmed use of chemical weapons." The letter was sent to two U.S. senators -- Republican John McCain and Democrat Carl Levin -- who had pressed the administration about Syria's possible use of chemical weapons.

In a briefing call for reporters on the matter, a White House official said the administration is using the intelligence assessments as a starting point to continue to investigate the matter in an effort to establish a credible set of facts that can be corroborated.

The official said Obama’s "red line" on the issue hasn’t changed, and added if the use of chemical weapons is definitely established, the president will consult with allies and the international community on all available options.

Earlier this week, an Israeli general alleged that Syrian government forces had used sarin more than once during the two-year conflict.

British Prime Minister David Cameron cited the "limited but growing" evidence that Syrian President Bashar al-Assad's regime is using chemical weapons, telling the BBC on April 26 that the evidence is "very disturbing" and would amount to a war crime.

Syrian government forces and rebels have accused each other of using chemical weapons. A UN team is yet to begin its investigation.

Based on reporting by Reuters, AFP, dpa, BBC, and RFE/RL's Heather Maher in Washington